1. Field of the Invention
The present invention relates to an outer ring and a clamp-type joint for vacuum apparatus using the outer ring, which does not cause cracking of an O-ring during use at a high temperature.
2. Background Art
A typical example of a conventional clamp-type joint for vacuum apparatus specified in JIS B8365 (ISO 2861/1) (“Dimensions and shapes of clamp-type joint for vacuum apparatus”, 1988, hereinafter referred to as “JIS standard”) is shown in FIG. 3. The clamp-type joint for vacuum apparatus 100 has a coupling 101 having a pair of parallel flat planes 111 and 111, a center ring 102, of which the outer circumference is a concave groove 112, inserted between the pair of flat planes 111 and 111 of the coupling 101, an O-ring 103 engaged with the concave groove 112 of the center ring 102, and a brim 113 of the coupling 101 which is fastened by clamps (not shown) to seal fluids in the planes 111 and 111 of the coupling 101 and the face of the O-ring 103.
A slight internal pressure (positive pressure) may be applied to the clamp-type joint for vacuum apparatus 100, even in the case in which the sealing object is a vacuum pipe. If an internal pressure is applied, the O-ring of the clamp-type joint expands outwardly, thereby inhibiting stable sealing performance and causing a fluid to leak. In the worst case, the O-ring may come off from the clamp-type joint, thereby inhibiting sealing. In order to obviate this problem and to inhibit outward expansion of the O-ring 103 due to internal pressure, an outer ring 105 with a U-shaped cross-section made of metal is provided around the circumference of the O-ring 103 in a manner to cause the O-ring 103 to be engaged therewith, as shown in FIG. 4. A clamp-type joint for vacuum apparatus 200 having this structure can inhibit outward expansion of the O-ring when internal pressure is applied in a pipe of a vacuum apparatus, thereby ensuring stable sealing performance. In addition, since O-rings used for clamp-type joints have improved heat resistance in recent years, the clamp-type joint used in pipes of vacuum apparatuses are used at a high temperature of 200° C. or more in certain occasions.
However, if the clamp-type joint for vacuum apparatus 200 equipped with an outer ring is used at a high temperature of 200° C. or more, the O-ring 103 may crack, resulting in sealing failure in the worst case.
An object of the present invention is, therefore, to provide a clamp-type joint for vacuum apparatus which can exhibit stable sealing performance at a high temperature of 200° C. or more without cracking of the O-ring and an outer ring used with the clamp-type joint.
In view of this situation, the inventors of the present invention have conducted extensive studies. As a result, the inventors have found that (1) the O-ring cracks at the parts in which a pair of top and bottom brims forming a U-shaped outer ring contacts (X in FIG. 5), (2) analysis using a finite element method (FEM) taking into consideration the coefficient of thermal expansion at a high temperature of 250° C. confirmed, based on the internal stress distribution of the O-ring as shown in FIG. 5, that a maximum large stress of 54 MPa is concentrated in the X parts, (3) due to the greater coefficient of thermal expansion of the rubber used for the O-ring than the coefficient of thermal expansion of metal used for the outer ring by several digits, the O-ring significantly expands outwardly, is pressed against the outer ring, and strongly pushes the brim of the outer ring at high temperatures, creating a large stress concentration, which results in cracking of the O-ring, and therefore, (4) if the brim has a shape of a convex sector on the side coming in contact with the O-ring, the stress concentration to that part can be reduced and the problem of O-ring cracking can be solved. These findings have led to the completion of the present invention.